Diodes under reverse bias exhibit low reverse currents until Zener breakdown occurs. These reverse currents are alternatively referred to as leakage currents or dark currents because they are present in the absence of light. These dark currents are a strong function of temperature of the diode and are proportional to the area of the n-p junction.
When a photodiode under reverse bias is illuminated, the current is comprised of a dark current and photocurrent contribution. The photocurrent contribution is proportional to the amount of light absorbed by the diode while the dark current is approximately constant for fixed reverse bias voltage. In those situations where the photocurrents are greater than or of the same order of magnitude as the dark currents, and the latter are fixed, then it is a fairly simple matter to subtract out the constant dark current. However, correcting for dark currents becomes problematic when temperature induced dark current fluctuations become significant, when systems require modulation of reverse bias potentials or when the light induced photocurrents are small in comparison to the dark currents.
Known methods for nulling dark currents using reverse biased photodiodes under varying reverse bias requires a chopper and associated detection circuitry. This has limited development of systems which make use of photodiodes whose spectral response is alterable by varying the reverse bias such as those employing the Franz-Keldysh effect or quantum confined Stark effect.
Another drawback to many compensation schemes is that they require a reference diode that is completely optically shielded thereby imposing design constraints.
U.S. Pat. No. 5,182,448 (Ohtsuka et al.) discloses a photocoupler with suppressed noise characteristics comprising a pair of photodiode detectors each connected across the input terminals of an op-amp in such a way that the photodiodes cannot be reverse biased. Ohtsuka teaches that the photodiode which serves as reference must be completely shielded from light.
U.S. Pat. No. 5,117,099 (Schmidt) discloses an ambient light rejecting quad photodiode sensor array with unbiased sensor and reference photodiodes for compensation located outside the area of illumination. This arrangement does not allow for use of photodiodes under reverse bias.
U.S. Pat. No. 4,916,307 (Nishibe et al.) discloses a light detection circuit based on a charge storage optical sensor comprising a photodiode in parallel with capacitor, instead of the more common and simple current sensor approach. Nishibe teaches that the reference diode must be shielded from light in order to provide dark current compensation.
Accordingly, it would be advantageous to provide a simple, cost effective and reliable method of detecting light intensity using photodiodes under constant or varying reverse bias conditions which avoids the need for complicated detection circuitry and completely shielding one of the photodiodes.